1. Field of the Invention
The present invention relates generally to electrode structures for use in electrochemical devices. More particularly, this invention relates to interlayers for protection of anodes and polymer electrolytes in electrochemical structures from deleterious reaction with one another, and methods for their fabrication.
2. Description of Related Art
Lithium is an attractive material for use as an electrode component in electrochemical devices, such as batteries and capacitors, due to its very high energy density and low equivalent weight. However, lithium is highly reactive in ambient atmosphere and with a variety of other electrochemical device components, in particular polymer electrolytes, and thus its incorporation into electrochemical devices requires special processing. Typically, lithium battery manufacture is conducted in inert environments in order to guard against degradation of lithium until it is hermetically sealed within a battery cell container.
Even with these precautions, lithium may detrimentally react with incompatible materials in the processing environment. To address these problems, some researchers have proposed that the electrolyte facing side of the lithium negative electrode be coated with a “protective layer.” Several types of protective layers and methods for producing them have been proposed. Prior methods have generally focused on the application of a protective layer to a preformed lithium electrode.
Some research has focused on “nitridation” of the lithium metal surface as a means for protecting lithium electrodes. In such process, a bare lithium metal electrode surface is reacted with a nitrogen plasma to form a surface layer of polycrystalline lithium nitride (Li3N). This nitride layer conducts lithium ions and at least partially protects the bulk lithium of the negative electrode from a liquid electrolyte. A process for nitriding lithium battery electrodes it is described in R&D Magazine, September 1997, p. 65 (describing the work of S. A. Anders, M. Dickinson, and M. Rubin at Lawrence Berkeley National Laboratory). Unfortunately, the surface of lithium is very rough making it almost impossible to nitride, and lithium nitride decomposes when exposed to moisture. While lithium metal batteries employ nonaqueous electrolytes, it is very difficult to remove all traces of moisture from the electrolyte. Thus, trace moisture will ultimately compromise the protective properties of the lithium nitride.
Other pre-formed lithium protective layers have been contemplated. Most notably, U.S. Pat. No. 5,314,765 (issued to Bates on May 24, 1994) describes a lithium electrode containing a thin layer of sputtered lithium phosphorus oxynitride (“LiPON”) or related material. LiPON is a single ion (lithium ion) conducting glass. It is typically deposited by reactive sputtering of a lithium phosphate in the presence of nitrogen. The nitrogen, however, attacks the lithium surface, thereby making the process of direct deposition of the glass film impossible. Work in the present applicants' laboratories has developed technology for the use of glassy or amorphous protective layers, such as LiPON, in active metal battery electrodes (see, for example, U.S. Pat. No. 6,025,094, issued Feb. 15, 2000, U.S. Pat. No. 6,402,795, issued Jun. 11, 2002, U.S. Pat. No. 6,214,061, issued Apr. 10, 2001 and U.S. Pat. No. 6,413,284, issued Jul. 2, 2002, issued to Chu and Visco, et al. and assigned to PolyPlus Battery Company).
Some contemplated lithium metal protective layers are formed in situ by reaction between lithium metal and compounds in the cell's electrolyte which contact the lithium. Most of these in situ films are grown by a controlled chemical reaction after the battery is assembled. Generally, such films have a porous morphology allowing some electrolyte to penetrate to the bare lithium metal surface. Thus, they fail to adequately protect the lithium electrode. Other examples of potential protective layers may include the deposition of polymer layers that involve solvents or monomers that are incompatible with lithium.
The existing approaches do not provide for the fabrication of a lithium electrode and polymer electrolyte laminate without a barrier layer that substantially impedes ion conductivity.
Accordingly, it would be desirable to provide a protective layer having high ionic conductivity between a lithium electrode and a polymer electrolyte. In particular fabrication processing and successful operation of active metals, such as lithium, as battery electrodes would be enhanced by the provision of such a protective layer.